New research using satellite data has uncovered a substantial shortfall in the estimates of methane emissions from oil and gas production reported in the annual Inventory of U.S. Greenhouse Gas Emissions and Sinks from the Environmental Protection Agency (EPA).

The study, led by Joannes Maasakkers, a former graduate student at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), found emissions from oil production were 90% higher than EPA estimates and emissions from natural gas production were 50% higher than EPA estimates in the latest inventory. These areas represented the largest discrepancies in the team’s findings.

"This is the first country-wide evaluation of the emissions that the EPA reports to the United Nations Framework Convention on Climate Change (UNFCC)," said Maasakkers, who is currently a scientist at the SRON Netherlands Institute for Space Research.

Methane emissions from the oil/gas sector in the contiguous U.S. in 2012. The figure shows the original EPA estimates for 2012 and the results from the SEAS research. Source: Harvard John A. Paulson School of Engineering and Applied Sciences  Methane emissions from the oil/gas sector in the contiguous U.S. in 2012. The figure shows the original EPA estimates for 2012 and the results from the SEAS research. Source: Harvard John A. Paulson School of Engineering and Applied Sciences

Today, the EPA reports only total national emissions to the UNFCC. These estimates are calculated based on processes and equipment. Methane emitted by a single gas pump, for example, is multiplied by the total number of operational pumps in the country to come up with an estimate of total emissions from gas pumps. Maasakkers pointed out that getting estimates for individual facilities using this method is complicated by the difficulty of accounting for all emission sources. "We know that a relatively small number of facilities make up most of the emissions and so there are clearly facilities that are producing more emissions than we would expect from these overall estimates," he said.

Prior to the latest SEAS study, Maasakkers and his colleagues worked with the EPA to map regional emissions of methane from various sources to simulate the way methane moves through the atmosphere. The SEAS research team developed a method to trace and map total emissions from satellite data to their source. By comparing those simulations to satellite observations from 2010 to 2015 using their transport model, the researchers traced the path of emissions from the atmosphere back to the ground. In this way, they were able to determine any discrepancies between the observations and the simulations.

"When we look at emissions from space, we can only see how total emissions from an area should be scaled up or down, but we don't know the source responsible for those emissions," said Maasakkers. "Because we spent so much time with the EPA figuring out where these different emissions occur, we could use our transport model to go back and figure out what sources are responsible for those under- or over-estimations in the national total."

The researchers plan to continue monitoring U.S. methane emissions with high-resolution satellite observations, as well as continue working with the EPA to improve emission inventories, with the hope of gaining more clarity on where emissions come from and how they are changing.

"It's important to understand these emissions better but we shouldn't wait until we fully understand these emissions to start trying to reduce them," said Maasakkers. "There are already a lot of things that we know we can do to reduce emissions."

The research was funded by the NASA Carbon Monitoring System (CMS) program. The paper is published in the journal Atmospheric Chemistry and Physics.

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